In a sound communication system, an encoded signal is sent over a transmission channel to a receiver, where the incoming signal is used by a speech decoder to synthesize sound. Channel errors can adversely affect synthesized speech provided by the receiver, and there are several related art methods for concealing such channel errors.
The most common method of concealing channel errors is to use cyclic redundancy check (CRC) to detect the errors in the most important bits and then perform bad frame handling in the speech decoder. This usually means replacing the erroneous parameters with the previously received good ones, or with slightly modified versions of the previous good ones.
However, sometimes, this simple method of error detection and bad frame handling is not enough to prevent very audible errors from occurring. There are basically at least two reasons for this. The first reason is that the error detection code used in the CRC is often not 100% reliable, and therefore some erroneous frames are not marked as bad in the channel decoder; these frames, called Undetected Bad Frames (UBF), are then used in the normal speech synthesis in the speech decoder. This situation can produce very loud and audible artifacts in the synthesized speech. The second reason for the occurrence of audible errors is that, even after erroneous parameters have been replaced by previously received error-free parameters, the subsequent bad frame handling is sometimes unsuccessful and may produce audible artifacts.
One typical related art method for concealing channel errors, in the situations just described, is to analyze the synthesized speech for atypical speech frames and then conceal those previously undetected or unsuccessfully handled bad speech frames. This type of related art system for analyzing and correcting the synthesized speech often involves concealment units in combination with the channel decoder (including error detection) and bad frame handling, but the concealment units can also be implemented standalone (i.e. separately from the channel decoder and bad frame handling).
A major problem with related art algorithms for concealing errors in the synthesized speech is that those algorithms unintentionally conceal some error-free signals too, thus causing audible artifacts despite concealment measures. This is because of the rather non-stationary nature of a speech signal, which makes it rather difficult to separate erroneous parts of the synthesized speech. Another problem with the related art is that the concealment methods typically involve little more than signal attenuation, which merely reduces the volume of an erroneous sound fragment. A typical related art algorithm is as follows.
As shown in FIG. 1, an encoded signal on a line 12 is sent over a transmission channel to a receiver 10, where it is provided to a channel decoder 14 for processing. The channel decoder finds and corrects data bit errors, and then provides a channel-decoded signal on a line 16, including a speech parameter signal on a line 18 and a bad frame indicator signal on a line 20. The bad frame indication can be based on any of various different suggestive factors (CRC, channel SNR, signal level, et cetera). A speech decoder with bad frame replacer 22 is instructed by the incoming bad frame indicator signal to entirely or partially replace erroneous parameters with previous good parameters. Subsequently, the speech decoder with bad frame replacer 22 provides a synthesized signal 24 to a signal error analyzer 26 which analyzes the synthesized signal for erroneous sound characteristics atypical of human speech (the typicality standards are not correlated to the quality of the transmission channel). If the signal error analyzer does find atypical sound characteristics, it provides a modification command signal on a line 30 to a synthesized signal modifier 28 which responds to the modification command signal by either concealing errors in the synthesized signal on the line 24, or by allowing the synthesized signal on the line 24 to pass through unchanged if the signal error analyzer did not detect any significant errors. The synthesized signal modifier's concealment of audible errors is accomplished by lowering the signal gain (attenuation), and sometimes by additional actions, resulting in a synthesized output signal on a line 32.
As mentioned, a problem with related art algorithms is that they sometimes unintentionally detect and conceal error-free signals too, thus causing artifacts that audibly degrade the quality of the output speech signal. Thus, the related art involves a tradeoff between two contrary goals: avoiding changes to any error-free signals, while ensuring changes to all signal errors. Whenever the related art seeks to accomplish one of these goals, it does so at the expense of reaching the other goal.
Another problem with the related art algorithms is the ineffectiveness of their methods for concealing audible errors. For example, if a channel error would have caused a whistling sound, then the subsequent corrective modification may still provide an audible whistling sound, albeit at a lower and less disturbing volume.